SLATE: How Much Radiation Do You Get From a Mammogram? Not much.

How Much Radiation do you get from a Mammogram? Not Much.
By Brian Palmer, Slate, Posted Thursday, Nov. 19, 2009, at 6:11 PM ET

The U.S. Preventive Services Task Force recommended Monday that women without risk factors for breast cancer wait until their 50s before going in for regular mammograms. That's because early screening yields almost 50 times as many false positives as true cancers, triggering unnecessary biopsies and anxiety, and early detection doesn't do much good in 85 percent of cases anyway.

The Task Force noted that the radiation exposure associated with mammography was a minor factor in their recommendation; on Wednesday, Slate's Darshak Sanghavi wrote, "It's possible the radiation from those mammograms may end up causing more cancers than they prevent." How much radiation is in a mammogram?

An average of 70 millirems—roughly the dose you'd receive from your normal, everyday environment over a period of two and a half months. Most of our lifetime exposure to radiation comes from radon produced by decaying uranium in soil and rocks. Enough of this gas gets trapped in your house to deliver about 200 millirems per year.

Food, which incorporates uranium from the soil, adds to this total, as do air travel, smoking, and radioactive atoms inside your body. But that's no reason to worry: Studies have shown no increased risk of cancer among people experiencing as many as 1,000 millirems per year of background radiation. (As a general rule, though, you're better off with less exposure.)

The dosage figures for environmental radiation refer to effects on the whole body, but radiation delivered to specific body parts can be more or less destructive. Tissues that generate new cells more rapidly, like the thyroid or bone marrow, are particularly vulnerable.

Breast tissue is somewhat unusual, since the rate of cell turnover varies with the amount of estrogen. As a result, the breasts are much less susceptible to radiation among post-menopausal women. (That's one reason why it might be a good idea for women to wait a few more years before starting with the routine exams.)

Still, the dosage number cited above—70 millirems per mammogram—refers to a total "effective dose" that's been adjusted to account for the relative vulnerability of breast tissue averaged over a woman's lifespan. (Men are not included in the calculation.) In terms of these adjusted rates, the mammogram is pretty mild compared with other medical tests. A spine X-ray delivers twice as much radiation, a common kidney procedure quadruple. If you needed a CT scan of your abdomen, you'd get 14 times more. (Of course, you may not be getting these procedures every year.)

So what are the chances that a few mammograms in your 40s will cause breast cancer? No one is really sure, because it's difficult to construct a study that separates the mammograms that caused cancer from the mammograms that merely detected it. But the consensus is that mammography alone is very unlikely to cause breast cancer.

The only thing researchers can say with any confidence is that early and regular mammography increases the risk of disease for those women who have had prior exposure to high doses of radiation—either through cancer treatment, frequent chest X-rays to monitor tuberculosis, or employment in the atomic weapons industry.

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Explainer thanks Penny Butler and Shawn Farley of the American College of Radiology.